The present invention is an optically clear platinum-group metal curable liquid silicone rubber composition having good physical properties. The composition comprises a vinyldimethylsiloxy-terminated polydimethyl(diphenyl)siloxane copolymer comprising about 1 to 20 mole percent diphenylsiloxy units and having a viscosity within a range of about 20,000 mPa.multidot.s to 100,000 mPa.multidot.s at 25.degree. C., vinyldimethylsiloxy-terminated polydimethyl(diphenyl)siloxane copolymer comprising about 1 to 20 mole percent diphenylsiloxy units and having a viscosity within a range of about 50 mPa.multidot.s to 10,000 mPa.multidot.s at 25.degree. C. and optionally comprising up to 3 mole percent vinylmethylsiloxy units, a treated reinforcing silica filler, and curing agents.
Polydimethylsiloxane polymers when cross-linked by common cure mechanisms such as hydrosilation can provide optically clear silicone elastomers. However such silicone elastomers typically have poor physical properties, particularly tear strength. Therefore, it is common practice to use a reinforcing filler such as fumed silica or precipitated silica to improve the physical properties of the silicone elastomers. Due to the difference in the refractive index of the polydimethylsiloxane polymers and the silica filler the resulting elastomer is no longer optically clear, but translucent in appearance.
There is a continuing commercial need for optically clear elastomeric materials for use in applications such as interlayers in safety glass, in pharmaceutical equipment, and in the preparation of gas permeable lenses such as contact lenses. Therefore, considerable effort has been devoted to the development of optically clear silicone elastomers having physical properties suitable for such applications.
One approach to making optically clear silicone elastomers having suitable physical properties, such as tear strength, is to match the index of refraction of the polyorganosiloxane polymers with that of the silica filler by varying the organic substituents of the polyorganosiloxane polymer.
Burdick et al., U.S. Pat. No. 3,341,490, teach peroxide cured blends comprising 60 to 70 parts of a dimethylvinylsiloxy endblocked copolymer consisting essentially of about 6 to 9 mole percent of methylphenylsiloxy units, about 0.1 to 0.2 mole percent of vinylmethylsiloxy units and the balance essentially dimethylsiloxy units; and 30 to 40 parts of a vinyldimethylsiloxy endblocked copolymer consisting essentially of about 15 to 25 mole percent of vinylmethylsiloxy units and the balance essentially dimethylsiloxy units; and silica reinforcing filler. The blends are reported to be optically clear.
Hartlage, U.S. Pat. No. 3,624,023, teaches a silicone rubber composition vulcanizable at ambient conditions consisting essentially of (A) from 1 to 50 parts by weight of a reinforcing fumed silica treated with bis(trimethylsilyl)amine, (B) hydroxyl end-blocked polymethylphenylsiloxane having from 15 to 45 mole percent phenylsiloxy units selected from the group consisting of diphenylsiloxy units and methylphenylsiloxy units and any remaining units being dimethylsiloxy units and (C) cure components.
Travnicek, U.S. Pat. No. 3,996,187, describes a method were the refractive index of a polydimethyl(diphenyl)siloxane copolymer or a mixture thereof is matched to that of a reinforcing silica filler to provide an optically clear silicone elastomer. The compositions were peroxide cured.
Travnicek, U.S. Pat. No. 3,996,189, teaches platinum cured reinforced silicone elastomers having improved physical properties comprising at least copolymers of aryl siloxanes and alkyl siloxanes in a ratio of aryl to alkyl substituents providing 6 to 16 mole percent aryl chosen to provide a refractive index matched to the index of refraction of a silica filler. Preferably two copolymers are used and each has methyl groups with 6 to 16 mole percent aryl groups. One of the two copolymers contains terminal vinyl groups and the other copolymer containing at least some hydrogen bonded to silicon atoms.
Another approach to making optically clear silicone elastomers having desirable physical properties involves replacing the reinforcing silica filler with a silicone resin as described in Polmanteer, U.S. Pat. No. 3,341,490. Polmanteer teaches peroxide cured optically clear silicone elastomer compositions consisting essentially of (1) 60 to 70 parts of a vinyldimethylsiloxy endblocked copolymer consisting essentially of about 6 to 9 mole percent of methylphenylsiloxy units, and about 0.1 to 0.2 mole percent of vinylmethylsiloxy units, and the balance essentially dimethylsiloxy units, (2) 30 to 40 parts of vinyldimethylsiloxy endblocked copolymer consisting essentially of about 15 to 25 mole percent of methylphenylsiloxy units, and the balance essentially dimethylsiloxy units, and a silicone resin.
Another alternative approach to making optically clear silicone elastomers having desirable physical properties involves making the reinforcing silica filler of a size such that they do not appreciably scatter light as described in Polmanteer et al., U.S. Pat. No. 4,418,165.
The objective of the present invention is to provide a platinum-group metal catalyzed liquid silicone rubber composition having optical clarity and good physical properties. This objective is achieved by the use of a platinum-group metal curable liquid silicone rubber composition comprising two vinyldimethylsiloxy terminated polydimethyl(diphenyl)siloxane copolymer compositions having a mole percent of diphenylsiloxy units and viscosities within defined ranges. The compositions have the good physical properties typically associated with platinum-group metal curing systems and with bimodal polymer systems having differing viscosities. In addition, optical clarity can be achieved over a range of mole percent of diphenylsiloxy units in the copolymers.